One type of a waveguide optical device having a buried diffraction grating includes a DFB laser made from compound semiconductors, for example.
In recent years, there are techniques to improve the laser characteristics of a DFB laser by varying the coupling coefficient, which determines the amount of feedback of a diffraction grating, along the direction of a cavity.
For example, there is technique to improve the stability of the longitudinal mode upon a higher power optical output, by reducing the coupling coefficient toward the center of the cavity, thereby reducing the hole burning in the longitudinal direction.
There are other techniques to prevent occurrence of the hole burning. For example, there is a technique to gradually reduce the width of the buried diffraction grating toward the center of the cavity. Also, there is a technique to gradually increase the width of the buried diffraction grating toward the center of the cavity. Furthermore, there is a technique to gradually increase the height of the buried diffraction grating toward the center of the cavity. Also, there is a technique to gradually reduce the height of the buried diffraction grating toward the center of the cavity.
In addition, there is a technique to increase the threshold gain difference or the gain difference between the main and side modes, using a structure wherein the coupling coefficient is increased at the center of the cavity but is reduced at ends, as compared to the center.
Furthermore, there are a techniques to narrow the spectral line width by increasing the length of the cavity, when a DFB laser is used as an FM modulation light source, by dividing the drive electrode into three parts along the direction of the cavity, and modulating the injection current of the center electrode.